1. Field of the Invention:
The present invention relates to mass spectrometry. In particular, the present invention relates to an apparatus for improving the signal-to-background ratio in mass spectrometry.
2. Discussion of Background:
The ionization source is an integral part of mass spectrometry instrumentation. Resonance-enhanced multiphoton ionization (REMPI), electron impact ionization (EI), chemical ionization, etc. are techniques available to the mass spectroscopist. These techniques have advantages and disadvantages depending on the particular application. Some molecules or atoms, particularly the noble gases, are extremely difficult to ionize, or the ionization efficiency is too low, requiring large samples for analysis.
In using a mass spectrometer to analyze a gas sample, traces of a previous sample may interfere with subsequent measurements. Molecules may remain within the system, adhering to the walls of the system and then desorb during subsequent analyses. These residual molecules produce the so-called "memory" effect, that is, a form of measurement "noise" or interference with a subsequent measurement caused by traces of previous samples. Memory effects are difficult to eliminate. The decreased signal-to-background ratio resulting from memory effects may invalidate or at least compromise subsequent measurements. They are particularly troublesome when analyzing small volume or low-concentration samples. Moreover, when the residual gas is the same as that being measured in a subsequent measurement, the effect is particularly troublesome. Hydrogen, nitrogen, water, and carbon dioxide molecules are known to be especially difficult to remove.
A number of techniques are available to improve the sensitivity of mass spectrometer measurements. The signal-to-background ratio can be increased by increasing the concentration of the sample. If a sufficient amount of the sample is available, this can be done simply by increasing the amount of sample gas introduced into the system. Vacuum pumps can purge the system between measurements to reduce the background signal. However, a vacuum purge may not remove molecules that adhere to the walls of the system. A neutral background gas may be pumped through the system between sample measurements to help detach and remove any residual molecules adhering to the walls.
Other techniques involve pulsing the sample gas into the vacuum chamber of a mass spectrometer. Kimock, et al. (U.S. Pat. No. 4,855,594) use sample gas pulses with a density high enough to substantially sweep residual background gas from the path of the pulse, thereby increasing the system's signal-to-background ratio for signal detection. The pulse frequency is adjusted to maintain a quasi-continuous flow of sample gas through the vacuum chamber.
Bursack, et al. (U.S. Pat. No. 4,201,913) describe a valve that pulses to admit small volumes of sample gas to an antechamber disposed between the sample stream and the high vacuum enclosure of a mass spectrometer. The duration and frequency of the pulses are controlled so that sample flow into the high vacuum enclosure remains essentially constant. Bursack (U.S. Pat. No. 3,992,626) uses pulsed gas samples to a mass spectrometer for use with atmospheric gases. The pulse duration and frequency are chosen so that the amount of gas admitted during each pulse does not exceed the removal capacity of an ion-getter pump in the interval before the next pulse.
Implementation of these techniques generally requires relatively large samples. It would be advantageous, particularly for small and difficult-to-ionize samples, to have a high-energy, pulsed ion source that could be activated by a small packet of gas produced by a pulsed valve. If desired, the sample could be diluted with a carrier gas such as argon, neon, xenon, and so forth. For a small, pulsed sample, the residual background molecules would not contribute significantly to the signal produced by the sample.